The present invention relates to the field of submerged membrane modules, systems and processes, which are useful for liquid separation, bioreactor, and water purification.
In a submerged membrane system, membrane modules are submerged in water, water flow from outside to inside the membrane under vacuum suction to be filtered. Compared to the conventional inside-out membrane filtration, submerged membrane filtration has lower transmembrane pressure, less energy consumption. Submerged membrane system can filter a feed solution containing much higher solid concentration than the conventional inside-out membrane system.
Membrane fouling is a major issue in submerged membrane systems. A great effort has been made to prevent membrane form fouling. These efforts include improving membrane hydrophilicity, cleanability, aeration, back pulse and back flush cleaning.
U.S. Pat. No. 5,944,997 describes a submerged membrane system having aerator located underneath the membrane module. Though this type of aerator can provide aeration, the holes of the aerator are often found to be blocked by the accumulation of suspended solid particles in feed solution. U.S. Pat. No. 6,708,957 attempts to address this issue by suspending the aerator on a pivot point, the aerator moves through an arc of pendulum, in an alternative design, the aerator rotates in a generally horizontal plane about generally vertical axis. Although this type of aerator can prevent air hole from plugging, it does not create sufficient aeration and liquid vortex flow, which will reduce membrane fouling.
U.S. Pat. No. 5,192,456 describes aeration system using mechanical agitating device under the plate frame membrane module to agitate suspended solid and disperse aeration bubbles, but it does not generate liquid vortex flow around membranes, because its rectangular geometry and the agitator is far away from the membrane module.
In order to remove a certain harmful waste in biological wastewater treatment process, aeration has to be temporarily stopped because some microorganism which convert harmful materials to harmless materials dislike oxygen. In this time period, membrane is easy to foul due to lack of aeration. The submerged membrane systems in the prior art are insufficient to deal with this situation.
In a commercial submerged membrane system, back flash and back pulse are often used to clean membrane. The control system for back flash and back pulse are very sophisticated, it often makes membrane system too expensive to use, compared to conventional water treatment method.
Membrane life time is another major issue. Violent aeration can dramatically reduce membrane life time. U.S. Pat. No. 6,354,444, describes braid reinforced hollow fiber membranes, which is stronger than self-support hollow fiber membrane, but it is much more expensive than the self-support hollow fiber membrane. There is need to make self-support hollow fiber more durable than its original form.
The present invention is aimed to address these issues alluded to in the previous paragraphs.